Improving the efficiency of breast radiotherapy treatment planning using a semi‐automated approach

Abstract Objectives To reduce treatment planning times while maintaining plan quality through the introduction of semi‐automated planning techniques for breast radiotherapy. Methods Automatic critical structure delineation was examined using the Smart Probabilistic Image Contouring Engine (SPICE) commercial autosegmentation software (Philips Radiation Oncology Systems, Fitchburg, WI) for a cohort of ten patients. Semiautomated planning was investigated by employing scripting in the treatment planning system to automate segment creation for breast step‐and‐shoot planning and create objectives for segment weight optimization; considerations were made for three different multileaf collimator (MLC) configurations. Forty patients were retrospectively planned using the script and a planning time comparison performed. Results The SPICE heart and lung outlines agreed closely with clinician‐defined outlines (median Dice Similarity Coefficient > 0.9); median difference in mean heart dose was 0.0 cGy (range −10.8 to 5.4 cGy). Scripted treatment plans demonstrated equivalence with their clinical counterparts. No statistically significant differences were found for target parameters. Minimal ipsilateral lung dose increases were also observed. Statistically significant (P < 0.01) time reductions were achievable for MLCi and Agility MLC (Elekta Ltd, Crawley, UK) plans (median 4.9 and 5.9 min, respectively). Conclusions The use of commercial autosegmentation software enables breast plan adjustment based on doses to organs at risk. Semi‐automated techniques for breast radiotherapy planning offer modest reductions in planning times. However, in the context of a typical department's breast radiotherapy workload, minor savings per plan translate into greater efficiencies overall.


2.A | Patient/plan data
The treatment plans for 40 clinical breast radiotherapy patients were selected consecutively from a reverse chronologically ordered list. All patients had consented at time of treatment for their images to be used for research purposes. Patients had previously been scanned using either a Philips Brilliance Big Bore (60-cm field of view, 2-mm slice thickness) or GE Lightspeed (50-cm field of view, 1.25-mm slice thickness) CT scanner. Outlining and planning were carried out on a Pinnacle 3 v9.8 treatment planning system. Scripting was performed using the planning system's inbuilt programming language, which incorporates object-oriented aspects. All patients were treated to a prescription dose of 40 Gy in 15 fractions with field energies of predominantly 6 MV; 10 MV was also used when necessary for patients with large chest wall separation.
The median (range) of breast volumes treated was 952 cc (223-2697 cc). Of these, 18 were left-sided treatments and 22 were right-sided. Eight patients received irradiation to the supraclavicular fossa.

2.B | SPICE
The autosegmentation software used was the Smart Probabilistic Image Contouring Engine (SPICE), a purchasable module for Pinnacle 3 . 14 The autosegmentation process applies rigid and deformable registrations together with probability-based structure refinements.
The modified atlas contours are subsequently added to the structure set. The suitability of the SPICE heart and lungs volumes for clinical use was investigated using a cohort of ten patients from the UK HeartSpare study. 15 The volumes created were compared quantitatively to those defined by an experienced radiation oncologist using the Dice Similarity Coefficient (DSC), which is twice the ratio of the volume of the overlapping region to the sum of the two volumes. A comparison between mean heart doses was also performed. 14

2.C | Scripting
A script was developed (PW, RAM) with the aim of automating parts of the breast planning process in order to facilitate reductions in step-and-shoot planning times while maintaining plan quality. The script workflow is given in (Fig. 1); it was not the purpose of this script to automate tangential field placement. The first part of the script initially created the prescription point at the isocenter of the breast fields and set a default prescription of 40 Gy in 15 fractions; the beam weightings were also set for each beam to give equal contributions to the dose at the prescription point. The planner had then to derive the necessary minimum, uncompensated, dose coverage of the whole breast by renormalizing the open tangential field distribution to achieve 95% dose coverage. has fixed jaws, MLCi has movable X and Y jaws and Agility only has movable Y jaws according to IEC 61217 convention. The script was initially created for MLCi only but its applicability was subsequently extended to encompass all three MLC types.

2.D | Planning
All planning was conducted by a single planner. The SPICE module was applied and plans were then created retrospectively using the scripts. All patients were further retrospectively planned manually and planning times were measured using a stopwatch. Dose calculations were performed using a collapsed cone convolution algorithm on a 0.25 9 0.25 9 0.25 cm 3 resolution dose grid. Plan statistics were extracted using the whole breast, field-based definition as per the IMPORT HIGH trial for the original and scripted plans. 17 The whole breast volume was created from the 50% dose level, contracted by 10 mm superior/inferior and 5 mm posterior. This volume excludes the lungs expanded by 5 mm and the skin, which is taken to be 5 mm from the external contour. Dose homogeneity was quantified by the homogeneity index, defined as the ratio of the difference between the breast D 2% and D 98% to the D 50% . 18 Tests for normality were undertaken by histogram and Q-Q plot inspection and the Wilcoxon Signed Rank Test was used to identify statistically significant differences between plan types.

3.A | SPICE
A summary of the comparison between the heart and lungs volumes created by the clinician and those added by the SPICE thorax atlas is given in (Table 1). SPICE creates two heart volumes (Heart 1 and Heart 2); according to Bzdusek et al., this was found to be necessary during the acquisition of the ground truth patient data that constitute the atlas. 14 More superior slices were outlined by the Heart 1 structure; however, the remaining slices corresponded exactly to those delineated by Heart 2. Heart 1 showed greater agreement with the clinician-outlined heart with a mean DSC of 0.92 compared to 0.82 for Heart 2. Furthermore, the median difference in mean heart dose was smaller for Heart 1 than Heart 2 (0.0 cGy and 2.4 cGy, respectively) when compared to the clinician-delineated heart. However, it should be noted that, owing to the limitations of out-of-field dose calculations, these statistics should be interpreted as estimates only. 19

3.B | Planning
Automated treatment plan quality was comparable to the original clinical plans in terms of dose homogeneity. No clinically relevant statistically significant differences were observed (P > 0.01) for the target parameters tested ( Table 2). The dose distributions given in (Fig. 2) were representative for the patient cohort.
The planning times achieved for the different MLC types following manual retrospective planning are given in (Table 3). For MLCi and Agility, utilizing the automated script reduced planning times by 45% and 36% of the respective manual planning times (P < 0.01). Reductions in maximum planning times were also observed (16.1 min for MLCi).

Volumes
Heart 1 Heart 2 Lung-L Lung-R Values are specified as median (range). The two SPICE-generated heart structures were compared to a single, clinician-defined heart for each patient. The aim was to reduce outlining and planning times while maintaining plan quality.
We have shown that SPICE gives clinically acceptable outlines for the heart and lungs (mean DSCs 0.92 and 0.97, respectively) and that the simple scripting solution allows for breast plans to be produced in shorter times whilst maintaining plan quality (6 min vs.  Values quoted are median (range). The level for statistical significance is P < 0.01. The SPICE Heart 1 structure is used for reporting.
modifications to the existing segments in order to produce a clini- It is inevitable that simple partial automation techniques will be superseded by more sophisticated commercial solutions. The method described by Purdie et al. and implemented in the RayStation treatment planning system is such an example. 13 Nevertheless, as we show with this study, there is a role for noncomplex, noncommercial techniques to provide planning efficiencies until it becomes feasible for commercial approaches to be implemented. On a per plan basis, the benefit of such decreases is relatively modest. However, in the context of the yearly departmental workload for breast planning, this small benefit has the potential to translate into much greater planning efficiency.

| CONCLUSION
The SPICE autosegmentation software offers a robust solution to automatic heart and lung delineation for breast patients and enables plan adjustment based on heart doses without introducing the burden of manual outlining. Partial automation of the breast radiotherapy treatment planning procedure through Pinnacle 3 scripting facilitates reductions in planning times without compromising plan quality.